In telephone systems there is a requirement that devices, such as modems, which are coupled to telephone lines must provide isolation between the device and the central office. Typically, prior art systems provide isolation by using a transformer between the telephone line and the device. The telephone line is usually connected to the primary side of the transformer and the device is on the secondary side. Signals from the transmission line and from the device are coupled across the transformer so that current does not flow directly from the device to the central office.
Instead of using a transformer to couple a device to a transmission line, some prior art systems use optically coupled circuits to provide the required isolation. This type of system is disclosed in U.S. Pat. Nos. 4,190,747 and 4,228,323, both to Feiner et al and in co-pending Patent application Ser. No. 08/957,916, assigned to a common assignee, the disclosures of which are hereby incorporated by reference. Optically coupled systems use photo emitters and photo detectors to couple the two sides of the circuit using light waves.
However transformers have disadvantages due to their physical size as well as their cost. Optocouplers are used for PC/MCIA applications where size constraints are severe. However, these circuits are very complicated and costly since they require feedback circuits to cancel errors due to component variation, especially the current transfer ratio (CTR) of the optocoupler. CTR, is the ratio of induced photo current in a photo-transistor to the forward current of an LED.
The circuit must provide a load to the telephone network consistent with certain requirements. These requirements are usually in the form of a permissible DC voltage and current. Additionally, the AC input impedance is specified as well, which typically is 600 Ohms in the U.S.
The electrical isolation provided by the circuit accomplishes two objectives: 1) isolates the common voltage or ground of the telephone line from that of the computer's allowing them to be substantially different and helps isolate; 2) voltage surges which may be present on the telephone line which could prove to be destructive to sensitive integrated circuits within the computer.
A final requirement for the circuit is that it performs a 4-wire to 2-wire conversion. From the figure, the "4 wires" are the receive signal (Rx) and its reference (typically the computer system's ground) and the transmit signal (Tx) and its reference. The "2 wires" are the tip and ring, which carry the standard signals which telephones and modems use.